[3.0.x] Enable PlayNettyServer in projects that require Http2 support

[mkurz]

Same as

• [3.0.x] Run tests with netty backend + mention netty backend in each README #526

however these three tests need http2 enabled, but Play's netty backend does not support Http2 yet... Someone started to work on this already but didn't finish: https://github.com/orgs/playframework/discussions/12004#discussioncomment-7409635

[ihostage]

I've seen the result of this work and it's very close to zero 🤷‍♂️
Moreover, I spent a some time to continue this work and understood that it isn't so easy as I think previously.
That's why I holded this activity in favor to more priority issues for us 😄
But I can share one idea that I caught then. Firstly we should invest time to support Netty in Play gRPC because it's really related with support HTTP 2 for Netty backend.